Boosters of this type comprise, in a known way, a vacuum chamber and a working chamber which are separated from one another by a sealed diaphragm borne by a skirt associated with a piston which is axially displaceable in one direction by a pressure difference between the two chambers and in the other direction by a return spring.
The booster piston is mounted between a control rod actuated by a brake pedal, and a push rod which drives a piston of a brake master cylinder (generally the primary piston of a tandem master cylinder).
At its end on the booster side, the control rod is terminated by a plunger guided in axial translation in a housing of the piston. This housing opens onto a reaction disk made of incompressible material, for example an elastomer or the like, which is interposed between the piston and the push rod.
A three-way valve is mounted in the piston around the control rod and comprises an annular shutter engaging with a sealing seat formed on the piston and with a sealing seat formed on the plunger. This valve is actuated by the movement of the control rod and controls the supply of atmospheric air to the booster.
At rest, in a non-braking condition, the working chamber of the booster is isolated from the external atmosphere by the shutter of the three-way valve and communicates with the vacuum chamber, itself connected to a vacuum source such as the intake manifold of the internal combustion engine of the vehicle. When the driver presses on the brake pedal, the control rod is moved toward the booster and actuates the three-way valve, the shutter of which closes off communication between the two chambers and opens a passage for supplying atmospheric air to the working chamber. Under the effect of the pressure difference which increases progressively between the two chambers, the piston of the booster moves the push rod toward the master cylinder in order to take up the mechanical play in the braking circuit during an initial braking phase known as the jump phase, and then, with the driver continuing to press on the brake pedal, the pressure increases progressively in the working chamber of the booster and the output pressure of the master cylinder increases linearly as a function of the force applied to the control rod until reaching a saturation phase at which the boost supplied by the booster is at a maximum and does not increase any further, the pressure in the working chamber being equal to atmospheric pressure.